1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display apparatus widely used for liquid crystal televisions, notebook personal computers and the like, a method for driving the same, and a method for manufacturing the same.
2. Description of the Related Art
An active-matrix liquid crystal display apparatus 1 as shown in FIG. 7 has been widely used for liquid crystal televisions, notebook personal computers, various kinds of information processors and the like. In the active-matrix liquid crystal display apparatus 1, liquid crystal 4 is sandwiched between an active-matrix substrate 2 and a counter electrode substrate 3. On the active-matrix substrate 2 and the counter electrode substrate 3, a pixel electrode 7 and a counter electrode 8 are formed on the surfaces of electrical insulating glass substrates 5 and 6, respectively. The light transmittance of the liquid crystal 4 sandwiched between the pixel electrode 7 and the counter electrode 8 changes according to the voltage applied between the electrodes, and an image can be displayed by controlling the applied voltage in accordance with the image. The counter electrode 8 opposed to the pixel electrode 7 is made of a transparent conductive material such as ITO. In a part of the surface of the counter electrode substrate 3, a black matrix (BM) 9 is formed. In the part of the surface of the active-matrix substrate 2 opposed to the part where the black matrix 9 is formed, a thin-film transistor (hereinafter, abbreviated as “TFT”) 10 is formed.
FIG. 8 shows an equivalent electrical structure of the active-matrix liquid crystal display apparatus 1 as shown in FIG. 7. The TFT 10 is formed at each of the intersections of gate signal lines 11 and source signal lines 12 on the active-matrix substrate 2. The gate signal lines 11 and the source signal lines 12 intersect at right angles, and supplementary capacitance lines 13 are also formed in parallel to the gate signal lines 11. That is, a plurality of gate signal lines 11 and a plurality of source signal lines 12 are formed so that the TFTs 10 and pixel capacitors (CLC) 14 formed between the pixel electrodes and the counter electrode are connected to the intersections of the gate signal lines Gn, Gn+1, Gn+2, . . . and the source signal lines Sn, Sn+1, Sn+2, Sn+3, . . . The gate signal lines 11 and the source signal lines 12 are electrically insulated from each other. To the gate signal lines 11, the gate electrodes of the TFTs 10 are connected, and to the source signal lines 12, the source electrodes of the TFTs 10 are connected. The drain electrodes of the TFTs 10 are connected to the pixel capacitors 14 and supplementary capacitances (Cs) 15. The counter electrode between which and the pixel electrodes the pixel capacitors 14 are formed is connected in common to common signal lines 16 on the counter electrode substrate 3 of FIG. 7. The other electrodes of the supplementary capacitances 15 are connected in common to the supplementary capacitance lines 13 on the active-matrix substrate 2 of FIG. 7. The supplementary capacitance lines 13 are connected to the common signal lines 16 outside the display area or at a peripheral circuit. The pixel electrodes form the pixel capacitors 14 through the layer of the liquid crystal 4, and form the supplementary capacitances 15 through a gate insulating film that electrically insulates the gate signal lines 11 and the supplementary capacitance lines 13 from the source signal lines 12. This structure is called a Cs on Com structure.
In the active-matrix liquid crystal display apparatus as shown in FIG. 8, a scanning signal is provided so that Gn, Gn+1, Gn+2, . . . of the gate signal lines 11 are selected one by one and only the TFT 10 connected to the selected gate signal line 11 is on. Methods of forming the supplementary capacitances Cs include a method called a Cs on Gate structure in which the supplementary capacitances Cs are formed between the gate electrodes of the TFTs 10 connected to the preceding gate signal lines 11 scanned immediately before, and the pixel electrodes. In the Cs on Gate structure, since the supplementary capacitance lines 13 are unnecessary, a large light transmission area can be secured. However, since the supplementary capacitances Cs are connected to the gate signal lines 11, the signal delay at the gates of the TFTs 10 is long. Therefore, the Cs on Com structure is frequently adopted for large-size active-matrix liquid crystal display apparatuses and for, even in the case of small-size apparatuses, high-resolution liquid crystal display apparatuses in which the density of the gate signal lines 11 is high.
In a method for driving the active-matrix liquid crystal display apparatus 1 as shown in FIG. 8, when writing to the pixels of the n-th line is performed, an on signal is input to the gate signal line 11 that is the gate line Gn of the n-th line. The on signal is provided at Vgh as the gate potential at which the TFTs 10 are brought into conduction. To the gate lines other than Gn, an off signal of Vgl which is the potential that drives the TFTs 10 into cutoff is input. Consequently, only the TFT 10 of the n-th line is conducting. At this time, a signal voltage at which the pixels of the n-th line are to be charged is supplied to the source signal lines 12. When the writing to the pixels of the n-th line is finished, the off signal is input to the gate line Gn, and the on signal is input to the next gate line Gn+1. By repeating this scanning, the pixel capacitors 14 corresponding to all the pixels can be charged at a given voltage value. Since the optical transmittance of the liquid crystal 4 of FIG. 7 changes according to the voltage applied to the pixel capacitors 14 formed by the liquid crystal 4 between the pixel electrodes and the counter electrode, a given image can be displayed by adjusting the amount of transmitted light from the backlight provided on the back surface of the active-matrix substrate 2.
In active-matrix driving, at each pixel, after a signal voltage is provided in one scanning, it is necessary to hold the potential during the one frame period to the next scanning. However, the provided potential cannot be held only by the pixel capacitors 14, and the pixel potential is changed by the leakage current of the liquid crystal 4, the off current of the TFTs 10, leakage of alternating components through a part of capacitor coupling between signal lines and the like. The change of the pixel potential at the pixel capacitors 14 results in degradation in display quality. To suppress the display degradation, the supplementary capacitances 15 are disposed in parallel to the pixel capacitors 14. Change of the potential difference between both ends of the pixel capacitors 14 can be reduced by providing the supplementary capacitances 15.
FIGS. 9A to 9C show the general outlines of signal waveforms that drive the gate signal lines 11 and the common signal lines 16 of the active-matrix liquid crystal display apparatus 1 shown in FIG. 7. Since the supplementary capacitance lines 13 are connected to the common signal lines 16, a Com signal is equivalent to a Cs signal. FIG. 9A shows a gate signal applied to the gate signal line 11. FIG. 9B shows a common signal applied to the common signal lines 16. FIG. 9C shows the gate signal and the common signal so as to be superposed on each other. When application of a direct-current bias to the liquid crystal 4 is continued, the display characteristic deteriorates. Therefore, for data signals supplied through the source signal lines 12, a driving method that reverses the signals every frame or every scanning line period is employed. FIGS. 9A to 9C show an example of a 1H reversal driving in which the signals are reversed every one scanning line period. The gate signal that disables the TFTs 10 is changed between two levels Vgl+ and Vgl−every scanning line period.
Japanese Examined Patent Publication JP-B2 6-46351 (1994) discloses, as a method for driving an active-matrix liquid crystal display apparatus, a structure in which the gate signal is switched between at least two levels every field in a period during which the transistor serving as the active-matrix switching element is nonconducting. This makes the influence of occurrence of defective display inconspicuous when the transistor is defective and the gate signal is directly applied to the pixel electrode.
Display methods of liquid crystal display apparatuses include a normally-white mode in which white display is provided when no voltage is applied across the liquid crystal and a normally-black mode in which black display is provided when no voltage is applied. Generally, the normally-white mode is frequently used in which a high contrast ratio can be secured and the control margin of thickness of the liquid crystal cell is large.
FIGS. 10A and 10B show the normally-white mode and the normally-black mode so as to be compared based on the correspondence between the voltage applied between the electrodes and the transmittance. In the normally-white mode, the transmittance decreases as the applied voltage increases. In the normally-black mode, the transmittance increases as the applied voltage increases. In each mode, the voltage at which the transmittance is 90% is a threshold voltage Vth.
The manufacturing cost of the active-matrix liquid crystal display apparatus 1 largely depends on the manufacturing yield. Therefore, preventing articles having a few defects from being regarded as defective articles as well as reducing defects caused in manufacture is important. Defects of liquid crystal display apparatuses include line defects that show up with respect to pixels arranged on a line and point defects that show up in units of pixels. The point defects are divided into bright points that are always displayed in white and black points that are always displayed in black. For example, for AV apparatuses such as liquid crystal televisions, since line defects and bright points are extremely conspicuous, even an article having only one line defect or bright point is regarded as defective. On the contrary, since black points are not very conspicuous, several black points are allowed.
The prior art of JP-B2 6-46351 is intended for making white point defects, that is, bright points based on active-matrix defects inconspicuous and preventing direct current from being applied to the liquid crystal to destroy the liquid crystal.
On the active-matrix substrate of the Cs on Com structure in which supplementary capacitances are provided for suppressing the change of the pixel potential between frames, leakage is apt to occur between pixel electrodes and the auxiliary electrode lines because of the structure. In a liquid crystal display apparatus that provides display according to the normally-white mode, when leakage occurs at a supplementary capacitance, the defect with respect to the pixel becomes a bright point, so that the manufacturing yield significantly decreases. JP-B2 6-46351 shows nothing as to measures against bright points associated with leakage of the supplementary capacitances. According to the method of JP-B2 6-46351, since a voltage such that “the potential of the counter electrode 8>the voltage in the off period of the gate line” is always applied to the liquid crystal, no effect of improving the reliability of the liquid crystal is obtained. (To improve the reliability, it is necessary to switch the polarity of the voltage applied to the liquid crystal layer.) Therefore, it is useless for the voltage of the gate signal in the second period to have not less than two levels. A method has also been proposed in which, to make bright points inconspicuous, correction is performed by use of a laser or the like to convert the bright points into black points or points that always display halftones. However, to perform the correction with reliability, it is necessary to previously dispose a correctable pattern, and disposition of such a pattern decreases the opening ratio at all the pixels, so that the image brightness decreases. In addition, since the step of the correction using a laser or the like is necessary and an apparatus for the correction such as a laser is necessary, the manufacturing cost increases.